Improved InAlGaP-based heterostructure field-effect transistors
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2006 Semicond. Sci. Technol. 21 540
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Semicond. Sci. Technol. 21 (2006) 540–543 doi:10.1088/0268-1242/21/4/021
Improved InAlGaP-based heterostructure
field-effect transistors
Y S Lin
1, D H Huang
2, W C Hsu
2, T B Wang
2, K H Su
2,
J C Huang
2and C H Ho
11Department of Materials Science and Engineering, National Dong Hwa University, 1, Sec. 2, Da Hsueh Road, Shou-Feng, Hualien 974, Taiwan, Republic of China
2Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, 1 University Road, Tainan 701, Taiwan, Republic of China
E-mail:[email protected] Received 16 January 2006 Published 7 March 2006
Online atstacks.iop.org/SST/21/540
Abstract
This investigation proposes the improved double δ-doped InGaP/InGaAs heterostructure field-effect transistor (HFET) grown by metalorganic chemical vapour deposition. The extrinsic transconductance (gm) and
saturation current density (Imax) of the double δ-doped InGaP/InGaAs
HFET are superior to those of the previously reported single δ-doped InGaP/InGaAs HFETs. The first n-InAlGaP/GaAs HFET is also
investigated because it has a high Schottky barrier, a large high band gap and a large conduction-band discontinuity (EC). Even without indium in the
channel of the InAlGaP/GaAs HFET, gmand Imaxare as high as
170 mS mm−1and 410 mA mm−1, respectively. The gmvalues of these two
HFETs remain large even when the gate voltages are positive. Moreover, the breakdown voltages of the two examined HFETs both exceed 40 V.
1. Introduction
AlGaAs/InGaAs high electron mobility transistors (HEMTs) have been established to display excellent power performance at microwave frequencies, but their noise figures remain inferior to those of InGaP/InGaAs HEMTs. The noise performance of InGaP/InGaAs HEMTs is improved by the absence of donor-related deep levels. AlGaAs contains DX centres, which are responsible for large threshold voltage shifts and drain I –V collapse at low temperature. Accordingly, the absence of deep traps in doped InGaP makes InGaP an alternative to AlGaAs in HEMTs. However, a heterostructure with a larger conduction-band discontinuity (EC) is required to provide a larger sheet charge density and reduce the knee voltage, which are essential for low-voltage power application. In 1996, Lu et al developed the first n-InGaP/GaAs heterostructure field-effect transistors (HFETs) [1]. Numerous researchers have presented a series of InGaP/InGaAs HEMTs [2–7]. Experimental findings have proven that a high-breakdown voltage can be achieved. Unfortunately, the two-dimensional electron gas (2DEG) densities of the InGaP HEMTs remain limited by their relatively small EC, which, in turn, limits their current-driving capability [1–7].
The In0.5(AlxGa1−x)0.5P/GaAs heterojunctions are excellent alternative materials for use in electronic and optoelectronic devices [8]. The In0.5(AlxGa1−x)0.5P/GaAs heterojunction with x 0.2 has a larger EC (0.25 eV) than the Al0.23Ga0.77As/GaAs (0.19 eV) or In0.5Ga0.5P/GaAs heterojunction. Although the value of EC for the In0.5Ga0.5P/GaAs heterojunction is still an issue of debate, evidence indicates that the incorporation of Al can increase both EC [9] and the band gap [10]. Hence, In0.5(AlxGa1−x)0.5P/GaAs HFETs with x 0.2 are expected to have a considerably larger 2DEG sheet density and current drive capability, and better carrier confinement of electrons because they have a larger EC. The other advantages of using In0.5(AlxGa1−x)0.5P as the barrier material in an HEMT structure include: (1) a higher breakdown voltage, because of a larger band gap, (2) the fact that the gate leakage current can be significantly reduced because the Schottky barrier is heightened and (3) the high selectivity of etching over GaAs, which improves the gate recess control.
This study develops double δ-doped InGaP/InGaAs and the first n-InAlGaP/GaAs HFETs. The δ-doped HFETs have a superior 2DEG concentration and a high-breakdown voltage [11–14]. For the InGaP/InGaAs HFET proposed
Improved InAlGaP-based heterostructure field-effect transistors Table 1. Hall measurements of this InGaP/InGaAs HFET and the previously reported GaAs/InxGa1−xAs structures (x= 0.25).
µn(cm2V−1s−1) n2DEG(1012cm−2) µn× n2DEG(1016V−1s−1) 300 K 77 K 300 K 77 K 300 K 77 K
Double δ-doped InGaP/InGaAs HFET (proposed herein) 5410 19 200 3.85 3.8 2.08 7.3 Single δ-doped InGaP/InGaAs HFETa 2100 7 100 3.8 3.4 0.8 2.4 Inverted δ-doped GaAs/InGaAs HFETb 3150 12 600 2.9 2.3 0.91 2.9 aReference [5].
bReference [13].
herein, symmetric double δ-doping is adopted to overcome the shortcoming of the low current capability in conventional InGaP-based HEMTs [1–5]. However, because InAlGaP/GaAs heterostructures have the advantages of high EC and high Eg, the n-InAlGaP/GaAs HFET is also investigated. Experimental results demonstrate that both these improved structures have better dc characteristics than the previously reported HFETs [1–8,17–19].
2. Device structure and fabrication
The two structures studied herein were grown by low-pressure metalorganic chemical-vapour deposition (LP-MOCVD). The InGaP/InGaAs HFET comprised a 0.5 µm undoped GaAs buffer layer, a δ-doped GaAs layer, a 100 ˚A undoped GaAs spacer layer, a 90 ˚A undoped In0.25Ga0.75As channel layer, a 100 ˚A undoped GaAs spacer, a δ-doped GaAs layer, an 80 ˚A undoped GaAs layer, a 400 ˚A undoped In0.5Ga0.5P Schottky layer, a 50 ˚A undoped GaAs setback layer and finally a 500 ˚A n+-GaAs cap layer sequentially grown on a (1 0 0)-oriented Cr-doped semi-insulating GaAs substrate. The growth temperature and pressure of the InGaP/InGaAs HFET were 690◦C and 150 Torr, respectively.
The InAlGaP/GaAs HFET was grown on a Cr-doped semi-insulating GaAs substrate{2◦off (1 0 0) towards [1 1 1]} as layers in the following order: a 0.3 µm undoped GaAs buffer layer, a 1000 ˚A n-In0.5(Al0.66Ga0.34)0.5P Schottky layer and a 750 ˚A n+-GaAs cap layer. A misorientated GaAs substrate was used to reduce the long-range ordering. The growth temperature and pressure of the InAlGaP/GaAs HFET were 725◦C and 150 Torr, respectively. Figures 1(a) and (b) display the approximate conduction-band diagrams of the InGaP/InGaAs and InAlGaP/GaAs HFETs. Trimethylindium (TMI), trimethylaluminium (TMA), trimethylgallium (TMG), arsine (AsH3) and phosphine (PH3) were used as the In, Al, Ga, As and P sources, respectively, to form the two structures of interest. Silane (SiH4) was adopted as the n-type source. The etchants for InAlGaP and GaAs were H3PO4:HCl (4:1) and NH4OH:H2O2:H2O (5:3:100), respectively. The etchant provides high selectivity of etching between InAlGaP and GaAs. Au/Ge/Ni metal was used to form source and drain ohmic contacts. Ag was then evaporated on Au/Ge/Ni to further reduce the contact resistance. Finally, Au was evaporated on the InAlGaP layer as the Schottky contact metal. The dimensions of the gate were 1.5× 125 µm2.
3. Results and discussion
The critical thickness (LC) of pseudomorphic strained layers must be determined to enable them to be grown. With
Au Gate EF i-InGaP δ-(n+)-GaAs EC i-InGaAs (a) (b) Au Gate EF n-InAlGaP Undoped GaAs EC EC ∆ φ φ
Figure 1. Conduction-band diagrams of (a) the InGaP/InGaAs HFET and (b) the InAlGaP/GaAs HFET.
reference to the proposed InGaP/InGaAs HFET, InxGa1−xAs has a different lattice constant from GaAs. The maximum thickness at which InxGa1−xAs can remain strained, without relaxation that would generate defects or dislocations, is called the critical thickness. The theoretical expression, proposed by Matthews and Blakeslee [15], for the critical thickness of the layer is ε= a 1−σ4lnLC· √ 2 a + 1 2·√2· π · LC· (1 + σ ) ,
where ε = 0.07x, a is the GaAs lattice constant and σ is Poisson’s ratio. The thickness is calculated to be 107 ˚A. Therefore, an InGaAs channel thickness of 90 ˚A is selected; this value is less than the critical thickness.
A Hall measurement is performed on the sample to determine the carrier mobility and the 2DEG concentration at 5000 G. The 2DEG concentration and mobility of InAlGaP/GaAs HFET are 4× 1012cm−2and 930 cm2V−1s−1, respectively, at 300 K. Table 1 lists the Hall measurement results that pertain to this InGaP/InGaAs HFET and the previously reported GaAs/InxGa1−xAs (x= 0.25) structures [5,13]. The InGaP/InGaAs HFET exhibits superior 2DEG concentration and mobility, because of the high quality of the GaAs/InGaAs heterostructure, the double δ-doped structure and the optimum spacer thickness of 100 ˚A.
Figure 2 shows the two-terminal gate–source I –V characteristics of the HFETs proposed herein at 300 K. 541
Table 2. Comparison of the characteristics of this double δ-doped InGaP/InGaAs HFET with the previously reported single δ-doped InGaP/InGaAs HFETs (Lg= 1.5 µm).
gm(mS mm−1) Imax(mA mm−1) BVgs(V) Double δ-doped InGaP/In0.25Ga0.75As HFET (proposed herein) 154 350 >40 Single δ-doped InGaP/In0.25Ga0.75As HFETa 82 215 >40 Single δ-doped InGaP/In0.15Ga0.85As HFETb 72 160 >40 aReference [4]. bReference [5]. -40 -30 -20 -10 -4 -2 0 2 4
Gate-Source Current Density (mA/mm)
n-InAlGaP/GaAs HFET InGaP/InGaAs HFET
T= 300 K
0 Gate-Source Voltage (V)
Figure 2. Two-terminal gate–source I –V characteristics of the InGaP/InGaAs and InAlGaP/GaAs HFETs.
-1.0 -0.5 0.0 0.5 1.0 1.5 0 50 100 150 200 T=300 K n-InAlGaP/GaAs HFET V DS = 4 V Extrinsic Transconductance (mS/mm) Gate-Source Voltage (V) InGaP/InGaAs HFET V DS= 5 V
Figure 3. Extrinsic transconductances versus gate voltage of the InGaP/InGaAs and InAlGaP/GaAs HFETs.
The InAlGaP/GaAs HFET exhibits an extremely low-leakage current, even the use of the n-doped Schottky layer. The gate–source breakdown voltages (BVgs) of the two studied HFETs, defined as the voltages at which the gate current density reaches 1 mA mm−1, exceed 40 V. Figure 3 plots the extrinsic transconductance (gm) versus the gate voltage of the InGaP/InGaAs and InAlGaP/GaAs HFETs at 300 K. The gmvalues of these two HFETs remain large, even when the gate voltages are positive. Therefore, an FET can be operated with a single power supply. The gmvalues of InGaP/InGaAs and InAlGaP/GaAs HFETs are 154 and 170 mS mm−1,
Table 3. Comparison of the characteristics of this n-InAlGaP/GaAs HFET with those of the previously reported n-InGaP/GaAs FETs (Lg= 1.5 µm).
gm Imax BVgs
(mS mm−1) (mA mm−1) (V) n-InAlGaP/GaAs HFET 170 410 >40
(proposed herein)
n-InGaP/GaAs HFETa 87.9 320 >40 n-InGaP/GaAs I-HEMTb 120 ∼250 26 n-InGaP/GaAs I2-HEMTb 100 ∼300 14 aReference [1].
bReference [2].
respectively. The Imax values of the InGaP/InGaAs and InAlGaP/GaAs HFETs are 350 and 410 mA mm−1, respectively. Table 2 compares this InGaP/InGaAs HFET with the previously reported HFETs of the same gate length [4,5]. This double δ-doped InGaP/InGaAs HFET proposed herein is superior to the single δ-doped InGaP/InGaAs HFETs. These facts demonstrate that the symmetrically δ-doped HFET provides a higher 2DEG concentration and mobility than the single δ-doped InGaP/GaAs HFETs. Table3 compares the characteristics of this n-InAlGaP/GaAs HFET with those of the n-InGaP/GaAs HFETs [1, 2]. The InAlGaP/GaAs HFET exhibits the more favourable gm, Imax and BVgs because the InAlGaP/GaAs heterojunction has a high ECand InAlGaP has a large band gap. Additionally, the examined InAlGaP/GaAs HFET exhibits better gm and Imax values than the other HEMTs [3, 4, 8, 16–19], even though this InAlGaP/GaAs HFET has a longer gate length. Moreover, the high-frequency characteristics are measured by an HP-8510C network analyser. The current gain cut-off frequency (ft) and maximum oscillation frequency (fmax) of the InAlGaP/GaAs HFET are 11 GHz and 23 GHz, respectively. ft and fmax of the InGaP/InGaAs HFET are 10 GHz and 18 GHz, respectively. These values are compatible with those of the previously reported HFETs with the same gate length [1].
4. Conclusion
This investigation proposes an improved double δ-doped InGaP/InGaAs and the first n-InAlGaP/GaAs HFETs grown by LP-MOCVD. These two improved structures provide the benefits of high drain current densities, high gm, low-leakage currents and high-breakdown voltages. These results demonstrate that the HFETs developed herein are appropriate for high-power applications.
Improved InAlGaP-based heterostructure field-effect transistors Acknowledgment
This work was supported by the National Science Council, Republic of China under contract no. NSC 94-2215-E-259-009.
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